Generally speaking, polyethylene has been successful in the field of foams since high pressure conditions lead to long chain branching, whereby the polyethylenes are characterized by high melt strength at considerably good processability. However, polyethylenes have various disadvantage and the application is severely limited. Polypropylene has very attractive properties such as high modulus, tensile strength, rigidity and heat resistance. However, the linear structure leads to poor processability making linear polypropylene unsuitable for numerous uses. Therefore, polypropylene for uses such as in foam is produced under conditions that result in long chain branching (LCB).
Thermoplastic foams posses a cellular structure generated by the expansion of a blowing agent. The cellular structure provides unique properties that enable the foamed plastics to be used for various industrial applications. Due to the attractive properties mentioned above and low material cost, polypropylene foams have been considered as a substitute for other thermoplastic foams in industrial applications. In particular, it can be expected to achieve higher rigidity compared to other polyolefins, higher strength than polyethylene and better impact strength than polystyrene. Furthermore, polypropylene allows a higher service temperature range and good temperature stability. However, polypropylene is suffering from some serious drawbacks, limiting its use for the preparation of foams. In particular, many polypropylenes have low melt strength and/or low melt extensibility.
Polypropylenes suitable for foam have been the object of various investigations in the past. Foam applications require high melt strength and at the same time good flow properties. The conventional and well known concepts for partially overcoming the drawbacks are use of high energy irradiation, treatment with peroxide, treatment with monomer/peroxide mixtures and solid phase treatment by subjecting polypropylene homo- or copolymer to a peroxide treatment in the presence of dienes.
Nevertheless, all these treatments result in various disadvantages. For example, the peroxide treatment in the presence of dienes leads to the formation of gels. Even worse, gel formation is usually increased when the extrusion screw speed reaches desirable industrial ranges. Other processes result in undesirably high crosslinking limiting the practical applicability of polypropylenes for foam.
Gel formation reflected by XHU usually results in undesirable low melt strength such as reflected by the F200 (cN) values. This problem is of high practical relevance since complete absence of gel formation cannot be achieved in industrial scale characterized by relatively high extrusion screw speeds.
Thus there is still the need for alternative or improved propylene polymer compositions being suitable for foam. Moreover, there is the need for propylene polymer compositions being suitable for foam having high melt strength and simultaneously low gel content also when extruded at high screw speed. Particularly, there is the need for a polypropylene polymer composition having high melt strength such as reflected by F200 (cN) even when containing low amounts of gels.